Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
Field of the Invention:
This inventio~ relates to an improved arrangement
for reducing voltage stress and corona discharge in high
voltage insulation and has particular, though not exclusive,
application to the equi-potential grading of high voltage
stator winding insulation of dynamoelectric machines.
Description of the Prior Art:
When an electrical conductor having one or more
sharp edges or corners is supplied with a sufficiently high
electrical potential, a discharge of electriclty known as a
corona discharge occurs between the edge of the conductor
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and the surrounding atmosphere. The corona discharge occurs
at the edges of the conductor because of the fàct that, for
a given potential, the electrical field intensity is greatest ;
at the sharpest edge and may re~ch a magnitude at which the
surrounding air breaks down while the intensity at other
parts of the conductor is still far below this magnitude.
The corona discharge thus limits the potential to which ~ :
the conductor can be sa~ely raised. This difficulty is
especially serious ln the case of an elongated conductor
10 which carries power at high voltage; i.e.~ a bus bar or a -
strip o~ rectangular cross-section, the ma~or sides of which
are substantially flat and parallel. It is customary to
llmit the corona discharge by rounding the sharp edges and
corners of the conductor and enclosing the conductor with
~ 1
an insulating material.
Corona deterioration at points of high voltage
stress ls one of the maJor causes of insulation failure in
high voltage appllcations. The corona itself is relatively
harmless; however, there are serious secondary effects which ;~
result from the production of powerful oxidizing agents in
an intense electrical field. Ozone is produced which ac-
celerates oxidation of ad~acent organic materials. Nitrogen
o-xide components, produced by the ioni~ation of the air,
combine with water to form acids that attack organic and
inorganic materials. Organic insulation~; such as varnishes
and cellulose are rapidly oxidized in a strong corona field.
Mica and glass are unaffected by corona and the oxidizing
agents because of their inert, inorganic composition.
Corona may be expected in dynamoelectric machines
which operate at levels of 6kv and greater. The corona
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discharge usually occurs in the hlgh voltage stator windings
within the stator slots and at the ends of the stator slots.
Slot corona occurs within the slots when the gas in the
small voids existing between the solid insulation and the
punchings ionizes. This has been overcome on some high
voltage machines by the application of a semiconducting
surface to the solid insulation. This semiconducting sheath
contains the voltage stress within the solid insulation, col-
lecting the capacitive current from along the coil surface
10 and discharging it harmlessly into the core; An example of ~-
A this approach is exemplified by Berg et ~l~Patent 3,210,461,
issued October 6, 1965, and assigned to the assignee of the
present invention.
End winding corona occurs where the coils leave the
slots and also between ad~acent coils in the end-turn regions.
There is a concentration of the voltage stress at the edge of
the core and corona may take place on high voltage windings
along the surface of insulation beyond the core or at the
end of the semiconducting treatment on the slot part of the
20 coils. It may also occur between-coils in-the end windings,
particularly-between-coils of-different phases where the
highest voltage stresses exlst. In order to grade the
voltage along the coil surfaces, it has been-customary to
apply a much hi~her resistivity surface treatment to a portion
of the coils ~ust beyond the core. This overlaps the lower
resistivity treatment an~ satisfact~rily distributes the
voltage gradient along-the coil surfaces into the end windings.
As operating voltages become increasingly greater,
it becomes more dîfficult to control the axial stre~ses by
~ 30 semi-conducting treatment alone, especially during over-
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potential testing. For these high voltages an alternate
~ethod of controlling the axial stress has been used. -
According to this method, conducting foils are incorporated
in the insulation wall, with or without semiconducting
surface treatment. ~y proper choice of foil spacing and
foil length, the radial and longitudinal stresses can be
evened out and the inæulation wall thickness reduced ap-
preciably. It is, therefore, a principal ob~ect of the
present invention to provide a conducting foil structure
which achieves maximum reduction in insulation voltage
~tre~s for a given number of conducting foilæ.
SUMMARY OF THE INVENTION
In accordance with this invention, an insulation
structure is provided which relieves voltage stress and
suppresses corona discharge in high voltage insulation
while substantially reducing the insulation thickness
required for a non-circular conductor operating at a
~peci~ied potential. For this purpo~e, one or more con-
ductive layers, or foils, are embedded within an insulation
structure of predetermined radial thickness. Each conductive
layer is uniformly spaced a predetermined distance from the
conductor so that the electrical stress at the surface of -
the conductor is equal to the electrical stress at the cor-
responding outside surface of each conducting layer. For
an insulated conductor having one or more curved edges, this
condition is realized in an insulation structure wherein
the spacing distance of a conducting layer irom the conductor
in the curved region of the insulation surrounding an edge
portion conductor edge and the radial thickness con-
ductor edge and the radius of the curved outer surface
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o~ the insulation in the curved region. If two or more
conductlng layers are employed, the equi-potential condition
is realized where the ratio o~ the ~pacing distances o~ 8UC- :
cessive conduc~ing layers are ln the same ratio as the spa-
cing distance of the first conducting la~er is to the radius;
of the curved conductor edge. Each conductlng layer so di~-
posed serves as an equi-potent~al surface which grades the -
voltage drop across the ln8ulatlng materlal, thereby xe-
duclng requlred lnsulation thickness ~or a given operatlng
potential and al80 reduclng or ellminatlng corona.
BRIEF DESCRIPTION OF THE DRAWINGS
ffl e lnventlon wlll be more fully understood irom
the ~ollowing detalled descrlption, taken ln connection with
the accompanylng drawlng, ln whlch:
Flgure 1 18 a partlal isometric view of a stator
slot section ~rom which two insulating electrical conductor
windlngs proJect,
Figure 2 18 a cross sectlon view of the electrical
conductor windings of Flgure l;
Flgure 3 1~ a cross section o~ an insulated con-
ductor windlng havlng a single conductive layer disposed
withln the insulatlon;
Flgure 4 is an approximate equivalent circuit
representatlon of the electrlcal conductor windings o~
Figure 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
m roughout the descrlption whlch ~ollows, like
reference characters refer to like elements on all ~lgures
of the drawing.
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Referring now to the drawing, Figure 1 is an
isometric view of a portion o~ a stator assembly 10 of a
dynamoelectric machine (not shown) having a rectangular
stator slot 12 extending axially along the stator and
-radially from the stator bore. The stator assembly 10 is
comprised of stacks of metal laminations having openings
aligned axially relative to each other so as to form the
axlally extending stator slot 12 in which electrical windings
and components of an insulating system in accordance with ~ -`
the present invention are placed. The stator assembly 10
includes electrical conductor windings 20 whlch are neces~
sary to obtain operation of dynamoelectric machines, which ~ -
is well known in the art. The eIectrical conductor windings
20 are insulated relative to each other and are insulated
. . .
relative to the stacked laminations of the stator assembly
10 in accordance with the present invention. The electrical
conductor windings 20 are confined within the stator slot
12 by means of a slot wedge 14 which is preferably composed
Fi6~ 9/ R 55 (~ t ~ 4J~ ~ ~r k) -:
of mica, asbestos, fibcrglasa or similar inorganic materials
with resin binding. A ^winding separator 16, preferably
micanite, is shown disposed between the electrical conductor
windings 20. The surface 29 of the insulated conductor
windings 20 within the stator slot-l~ and for a predetermined
1~ .
distance outside the slot is made semiconducting, preferably
by applying a stress-grading compositi~n-of silicon carbide.
¦ However, other coating materials may be employed.
The electrical conductor windings 2~-are shown to
have rectangular cross section with rounded corners. Al-
though a solid inner conductor 22 having rounded corners of
radius "r" is depicted, the conductor is commonly subdivided
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into a large number of mutually insulated conductor strands
in order to reduce the skin-effect losses. For convenience
of reference in this discussion, however, the inner conductor
22 will be treated as if it were a solid bar of rectangular
cross section having rounded corners of specified radius "r".
The inner conductor 22 is preferably composed of a highly
conductive metal such as copper or aluminum and is intended
to pass electrical currents from one point to another within
the dynamoelectric machine at high voltages. As a rule, the
dimensions of such conductors stand in high numerical ratios
to each other, the length being much greater than the width. -~
Unless the sharp corners of such a ~onductor be somewhat ~ -
rounded, a corona discharge may take place when the con- `
ductor is operated at a high potential.
A general outline of the components of the in-
sulating structure for the electrical conductor windings -
20 as disclosed by thls invention is illustrated in the end
view shown in Figure 2 and can be better understood by refer-
ring to Figures 3 and 4. A cross section of the electrical
conductor winding 20 is shown-in-Figure 3, and an approximate ~ :
equivalent circuit of the conductor assembly is shown in
Figure 4. Surrounding the inner conductor 22 is a first
layer 24 of insulating material which is built up to a
radial thickness "x" at each corner of the electrical conduc-
tor winding. This layer is preferably composed o~ mica split-
ting tape or mica paper. ~ -
In the present invention, a decrease in the high
electrical stresses at the edges of the conductor is achieved
by embedding one or more conducting layers within the insula~
tion. Accordingly, a conducting layer 26 is disposed between
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., . ~ . ; ~, , : . . . . .. . ; .
... . . ~ , , ;, ,. , , . , . . ., .,,.
., . . . ., . , - ., ..... . , . . . ,.. . ~ . , . ... , :
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the first insulation layer 24 and a second insulation layer
28 in a concentric relationship with inner conductor 22.
The conducting layer 26 is spaced from the surface of elec-
trical conductor 20 in a manner which will be described be-
low. The conducting layer 26 is in intimate contact with
the first insulation layer 24;and the second insulation layer
28, completely surrounds the insulation layer 24 and the in-
ner conductor 22, and extends axially at least as far as the
outer semiconducting surface 29 on the conductors 20. As
10 shown in Figure 3, the total radial thickness of both in- ~ -
sulating layers including the conducting layer thickness ln ~-
the curved regions is designated "T". The conducting layer
26 divides the combined thickness of both layers of insulation ~ -
ln the same ratio at the flat surfaces as around the curved
edges of the inner conductor 22, i.e.,
r + x = r + T -
r r + x
In Figure 3, oapacitors Cl, C2, C3 and C4 are shown
superimposed upon a cross section of electrical conductor
winding 20. An approximate equivalent circult of the as-
~0 sembly which shows the interconnection of these capacitors
is shown in;Figure 4. Cl represents the distributed capac- -
itance between the conducting layer 26 and the flat surfaces
of the inner conductor 22, C2 is the dlstributed capacitance
between the conducting layer 26 and the flat outer surfaces of
the insulation 28, C3 is the distributed capacitance between
the conducting layer 26 and the edges of the inner conductor
22, and C4 is the distributed capacitance between the con-
ducting layer 26 and the outer edges of the insulation 28.
The symbol R is the resistance of the conducting layer 26
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104~)696 ~
between the flat areas and the curved edge areas. The ~ .
symbols Vl and V2 represent the po.tentlal of the conducting ; :
layer 26 at its flat portions and its curved portions, res~
pectively, with the outer insulation surface as reference.
As mentioned above, the voltage stress at.. the . ;
edges of the inner conductor 22 for a given applied voltage
is higher than the uniform.stress in the flat parts of
the insulation. With no conducting layer present, the
voltage drop over C3 will there~ore be larger than the
10 voltage drop over Cl, i.e., Vl> V2. With the presence .
of a conducting layer 26 having a resistance "R" between
the flat portions and the curved edge areas that is low :
compared with the reactance of either of the four capaci- :~ -~
tances, the two voltages will be almost identical and be- ~ ~ .
cause Cl is greater than C3, and C2 is greater than.C4, the ~ ~:
two voltages will be closer to the initial value of Vl than :;
to the inltial value of V2. The result is that the stress .
over C3, and consequently the stress over the first insula-
tion layer in the curved region, has been relieved. ~ ~ -
An example.will now be given... Let the flat sur~
faces previously referred to be two inches and three inches :.
wide, respectlvely and the radius of the conductor.edge
r = 0.03 inches, the insulation thickness T = 0.30 inches,
and the conducting layer located at a distance of x = 0.10 `."
inches from the surfaces of the con~uctor. If the dielectric : ~`
:~ constant is assumed to be 4 (the choice does not effect the
result), the capacitances per inch of conductor length will
be~
C = 0.2248 10 x 4 = 89.92 pF
0 . 100
C2 = 0.2248 10 x 4 = 44.96 pF
0 200 :.
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C = 0.6137 x 4 = 3.85 pF
3l 0---130
0.030 ~ -
C4 = 0.6137 x 4 = 6.o6 pF
log 0.330
0.130
In the determination of C3 and C4 as well as the
following calculations, it has been assumed that the edge
of the insulation surface and the edge of the conducting
layer are parts of concentric, circular cylinders.
With 30kv applied to the conductor (average stress
lOOvpm), and with no conducting layers, the following values
for Vl and V2 are obtained: - -
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Vl = 3 Cl = 20~kv
lOC1+C2
V2 = 30 3 = 11.65 kv
3 4
The inner thlrd of the insulation at the edges has
therefore an average stress of:
1`~8350 = 183.5 vpm,
100 . :: :
and from standard charts, e.g., that published by J. D.
Cockcroft in "The Effect of Curved Boundaries on the Dis~
tribution of Electrical Stress of Round Conductors," J. Inst.
Elect. Engrs., Vol. 66, April 1928, pp. 385-409, the stress
at the edge of the conductor is:
2.57 x lO0 = 257 vpm
With a highly conducting layer such as conducting
;; layer 26, the voltage of the conducting layer becomes
V = 30 Cl+C3 = 19.43 kv
l 3
The inner third of the insulation at the edges now ~ -
has an average stress of only:
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10570 = 105.7 vpm,
100 ~,. . .
and the maximum streæs at the edge of the conductor i8 now
only:
1.80 x 105.7 = 190.3 vpm
It should be noted that these stresses will be
slightly higher when lt i~ taken into account that the con- -
ducklng layer has a certain resistlvity, but the difierence
wlll be small as long as the reslstance of the layer is small
compared with the impedance of the largest capacitance of the
sy~tem. In the example above, the impedance of Cl with 60
cycle excitation is:
377 x 89.92 x 10 12 = 2 9 x 107 Q
If the conductlng layer 26 is moved further in to-
wards the inner conductor 22, the stresses at the edge of
inner conductor 22 wlll decrease, while at the same time,
I the stresses at the outside surface of the conductlng layer
1 26 in the curved region will increase. The lowest maximum
I stress in the ln~ulation will exist when these two stresses
are the same, which accordlng to known principles of electro-
- 20 Qtatic~ will occur when the ~pac~ng distance from the inner
conductor of the conducting layer in the curved edge area is
equal to the geometric average of the radius of the conductor
edge~and the radial thickness of the insulation. The effect
o~ the conductlng layer is to make the average electric
stre8s the 8ame for the various parts of the ln~ulatlon as
it i8 dlvlded by the conducting layer 26.
If two or more conducting layers are utillzed~ the
maxlmum stress ln a given paxt of the insulation between two
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adjacent conducting layers will depend upon the ratio of
the spacing distances of these layers, and wlll increase as
this ratlo increases. In order to keep the h~ghest electric
stress in the insulation as low as possible with a given
number o~ conductinglayers, the hlghest ratio between the
spaclng distances oi two ad~acent conducting layeræ should
be as low as po~sible. This ls accomplished by locating the
conductlng layers such that all the ratios of spacing dis-
tances of ad~acent layers are the same; i.e.,
lOr + x r + x2 r ~ xi r + T
1 = = ,, = = =
r r + xl r + Xi-l r + xn
where r = radius of conductor;
T = radial thickness of insulation ln curved region;
Xi = distance from conductor sur~ace to the ith
conducting layer;
n = nu~ber o~ conducting layers (a posltive lnte-
ger); and
1 = a posltive integer less than or equal to n.
From thls relatlon it can be shown that
n+l
xl = l rn~l-i )i - r,
where n~i o. I~ only one conducting layer i8 u~ed,
n - l and
Xl = ~ - r.
The ~ormer equation may be expressed in the ~ollowing form:
(Xi + r) n+l ~ rn+l-i (r~
m e conductlng layer 26 is easily applied if the
insulatlon is built from multiple layers Or tape, as has been
suggested, because it may be conveniently inserted
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between those two tape layers which will give the be~t value
of x as determined from the equations given above. For axample,
in the single conducting layer ~tructure of Figure 3, for
a conductor ha~ing edge radius of 30 mil8, and insulatlon
thickness of 0.145 inches, the loweæt value of the maximum
stress i8 obtained when the conductlng layer is located
42 mils from the conductor surface. In a fully loaded
insulation system, a metal foil can be used for the con-
ductlng layer, but for an lnsulation system that i8 to be
vacuum impregnated, the conductlng layer must also be im-
pregnable. A conducting, impregnable tape i8 available, -~
has been tested as an outer binder tape, and ls also
suitable for use as a conducting layer ln a vacuum impreg- ~ -
nated insulation system.
Referring agaln to Figure 1, the conducting layer
26 and lnsulation layers 24 and 28 are shown extending
along the inner conductor 22 as lt pro~ects out of core slot
12. The conducting layer~ are extended out from the end of
a sem1conductlng outer sur~ace 29 on the conductor 20 a pre-
determlned distance in order to provide axlal grading on thesur~ace of and in the outer layers o~ the conductor insula-
tion in addition to radial gradlng.
It i8 therefore apparent that the structure des-
crlbed above 18 an improved arrangement for reducing radial
voltage stress in high voltage insulation wlth or without
slmultaneous reduction of axlal voltage stress. The struc- :
ture reduces or elimlnates lnternal corona di~charge while
substantially reducing the lnsulatlon thlckness requlred for
a specified operating potential. In addition to stator wind-
39 ings~ the gradin8 principle described herein can be used for
the lnsulation o~ any current carrylng metallic part having
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a non-circular cross section. It should be understood that
various modifications, changes and variations may be made -
in the arrangements, operations and details of construction
of the structure disclosed herein without departing from the
spirit and scope of the present invention.
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